The present disclosure relates to semiconductor device manufacturing, and more particularly, to optical proximity correction in lithography and verification of circuit layout.
The fabrication of integrated circuits on a semiconductor substrate typically includes multiple photolithography steps. A photolithography step is the image transfer step, which transfers a circuit layout through photo-mask to a silicon wafer. A photolithography process begins by applying a thin layer of a photo-resist material to the substrate surface of a silicon wafer. The photo-resist is then exposed through a photolithography exposure tool called stepper or scanner to a radiation source with wavelength in DUV range that changes the solubility of the photo-resist at areas exposed to the radiation. The photo mask, which contains circuit layout information, consists of a patterned material or materials that interact with the exposing radiation through intensity and/or phase modulation.
To improve an integrated circuit (IC) functionality and performance, IC manufacturers normally shrink the circuit components and at the same time, increases the number of circuit components. It becomes necessary to reduce the size of the features, i.e., the lines and spaces that make up the circuit elements on the semiconductor substrate. The minimum feature size that can be accurately produced on a substrate is limited by the ability of the fabrication process to form an undistorted optical image of the mask pattern onto the substrate, by the chemical and physical interaction of the photo-resist with the developer, and by the uniformity of the subsequent process (e.g., etching or diffusion) that uses the patterned photo-resist.
When a photolithography system attempts to print circuit elements having sizes near and below the wavelength of the exposing radiation, the resulting shapes of the printed circuit elements become significantly different from the corresponding pattern on the mask. For example, line widths of circuit elements may vary depending on the proximity of other lines. The inconsistent line widths can then cause circuit components that should be identical to operate at different speeds, thereby creating problems with the overall operation of the integrated circuit. As another example, line ends tend to shorten or “pull back.” The small amount of shortening becomes more significant as the lines themselves are made smaller Furthermore, pulling back of the line ends can cause connections to be missed or to be weakened and prone to failure.
Accordingly, Optical Proximity Correction (OPC) was developed to address lithography distortions in semiconductor manufacturing. The goal of OPC is to produce smaller features in an IC using given equipment set by enhancing the “printability” of a wafer pattern. In particular, OPC applies systematic changes to photo-mask geometries to compensate for nonlinear distortions caused by optical diffraction and resist process effects. For example, these distortions include line width variations dependent on pattern density that affect a device's speed of operation, and line end shortening that can break connections to contacts. Causes include reticule pattern fidelity, optical proximity effects, and diffusion and loading effects during resist and etch processing. A mask incorporating OPC is thus a system that seeks to negate undesirable distortion effects during pattern transfer.
OPC works by making small changes to the IC layout that anticipate the distortions. To compensate for line end shortening, the line is extended using a hammerhead shape that results in a line in the resist that is much closer to the original intended layout. To compensate for corner rounding, serif shapes are added to (or subtracted from) corners to produce corners in the silicon that are closer to the ideal layout. Determining the optimal type, size, and symmetry (or lack thereof) is very complex and depends on neighboring geometries and process parameters. Moreover, a sophisticated computer program is typically necessary to properly implement OPC.
However, applying OPC and verifying the result of OPC are not trivial endeavors. The detection of defective shapes that require OPC is very time consuming considering the huge number of electronic components and even larger number of shapes on a photo-mask.